Most probably derived from UK Fluid Ounce ( 28.41 millilitres) from India's British colonial past as 1 peg = 1 Ounce.
Now, presumably for convenience, one peg measure equals 30 millilitres.
The terms "large peg" and "small peg" are used, equal to 60 mL and 30 mL, respectively, with "peg" simply referring to a small peg.
A peg is informally considered as an amount of alcoholic drink that is taken at a single shot.
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